Many parts of the world experienced extreme weather as the planet Earth continued its seasonal march into the new year of 2014 . Winter in the Northern Hemisphere was a wildly fluctuating one (Figure 1). Blasted by frigid air from the polar region, temperatures in many states in the central, eastern and southern parts of the United States were well below freezing. Yet Alaska and the Arctic were abnormally warm. Much of Europe was also exceptionally mild in early January, with France reporting daily maximum temperatures five to nine degrees above the normal values of January. Summer in the Southern Hemisphere, on the other hand, was sizzling hot. The central and eastern interior of Australia was under the grip of a heat wave in early January, and this after Australia reporting a record hot year in 2013.
Figure 1 Air temperature (near ground) anomalies for 5-7 January 2014 over the Northern Hemisphere (the Arctic region at centre) relative to the 1981-2010 average; blue is colder and red is warmer (source: US National Oceanic and Atmospheric Administration).
It is recognized that climate change can lead to anomalous variations in weather patterns and the likelihood of more extreme weather. Some studies suggest that a warming climate can actually expose the mid-latitude region of the Northern Hemisphere to more incursion of cold air from the Arctic. While this may sound self-contradictory, there is actually a good physical reason behind.
Warming causes the melting of Arctic sea ice and snow cover on land areas, altering the reflectivity of the Earth. Sea ice and snow have much higher reflectivity than ocean and land, and hence are very effective in reflecting sunlight back to space. However, the declining sea ice and snow cover will expose more ocean and land surfaces, increasing the Earth's ability to absorb solar energy. The additional heat will warm the ocean and land, prompting further melting of sea ice and snow cover in a vicious cycle. As a result, the Arctic warms faster than elsewhere in the Northern Hemisphere. The temperature contrast between the polar region and the tropics is reduced, causing the westerly airstream aloft in the Northern Hemisphere to slow down and become more susceptible to wavy meanders. On the west flank of the wave, cold air from the polar region is drawn southwards by the northerly airstream; on the east flank of the wave, warm air from the tropics under a southerly airstream pushes towards regions further north (Figure 2).
Figure 2 Winds aloft (about 5 km above ground, unit of wind speed in m/s and indicated by shading) in the Northern Hemisphere on 5-7 January 2014 (source: US National Oceanic and Atmospheric Administration).
The slow-down of the westerly airstream aloft also favours the establishment of atmospheric blocking in the form of slow-moving anticyclones. The evolution of weather systems then becomes locked into a certain pattern in time and space. Depending on where you are with respect to the blocking set-up, some locations may experience prolonged fine weather, while others may be trapped under a storm corridor in a spell of rainy and gloomy conditions. If blocking occurs in winter, unusually cold or mild weather may persist for days or weeks over the affected regions. One example in recent years that comes to mind is the cold spell experienced by most part of China in early 2008, with cold air from Siberia taking advantage of the time afforded by a blocking situation to spread incessantly southwards all the way to Hong Kong.
Of course, for one single extreme weather event, we can never be sure that it is attributable to climate change. Only a collection of such events in some kind of trends over a period of time can tell the full story. As such, there is always a temptation to just maintain a watching brief, in the hope that the climatologists may turn out to be wrong or, failing that, in the misguided belief that the human race always has the capacity to adapt to climate changes, especially if the full impact we are talking about is, may be, a hundred years or so down the line. But what if the fury of climate change can also be manifested in the form of more frequent extreme weather? We certainly do not want to react too late if the next extreme weather event to hit us is just around the corner!
Are you aware of the scientific consensus on global warming?
The Intergovernmental Panel on Climate Change (IPCC) has just announced its Fifth Assessment Report (AR5) in September 2013. This report is made available to the public on a brand-new webpage (www.climatechange2013.org). There, at the bottom of the webpage, are three big and conspicuous numbers: "259", "39" and "54677" (Figure 1). What do they mean?
These numbers means that AR5 is authored by 259 scientists from 39 countries around the world with 54677 comments examined. In a factsheet from the website, it was further mentioned that over 9200 scientific papers were cited in the report. From all these numbers, we can see that IPCC's AR5 is compiled based on huge amount of peer-reviewed scientific evidence, representing climate scientists' consensus on the climate change issue. One important message from AR5 is that it is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century. The certainty of human's influence on climate has increased from 90% (very likely) in the previous report published in 2007 to 95% (extremely likely) in the current report.
The above scientific consensus on the human-caused global warming is affirmed by independent surveys. Released recently in May 2013, the Consensus Project (theconsensusproject.com) is one of such surveys. In this survey, researchers examined the abstracts of more than 10,000 peer-reviewed scientific papers in the last 21 years and found 97% of the papers expressing a position endorsing the human-caused global warming assessment. The researchers also directly enquired those authors who have expressed a position on global warming and found that 98% of the authors confirmed their support of the view.
As a matter of fact, more than 30 science academies, scientific organizations and research institutes around the world have clearly expressed their endorsement of human-caused global warming . Then why do we hear deniers claiming the non-existence of scientific consensus every now and then? The reason is simple: if the public perceives that scientists are still arguing, they will believe that it is still an unsettled issue and play down the urgency of the situation. Lets hear one story. In early 2012, a group of "scientists" wrote on the Wall Street Journal (WSJ) claiming that there was a growing number of "scientists" disputing the fact of global warming. Five days later, some 30 climate experts rebutted on WSJ, revealing that most of the so-called "scientists" had no expertise in climate science .
So, in light of the above, what would you believe?
Figure 1. New webpage for the Fifth Assessment Report of IPCC Working Group I
Wednesday, 18th December 2013
Why is my home colder?
Under the influence of the winter monsoon, the weather remained fine and cool in Hong Kong from late November to early December this year. You might have heard of weather reports like "the minimum temperature in the morning will be certain degrees in the urban areas, and a few degrees lower in the New Territories" during these days. For example, while the minimum temperature recorded at the Observatory in the morning of 5 December was 15.8 degrees, those over parts of the New Territories such as Ta Kwu Ling, Pak Tam Chung, Shek Kong and Sheung Shui were all below 10 degrees (Figure 1). What caused the large regional difference in temperatures within such a small place like Hong Kong?
Figure 1 Minimum temperatures at various locations of Hong Kong in the morning of 5 December 2013.
Generally speaking, the ground releases heat to the sky at night in the form of infrared radiation. The temperature of the ground drops as a result of the loss of heat. Since heat is transferred by way of radiation and does not require any matter to serve as the medium, this weather phenomenon is known as "radiation cooling". Its effect is most prominent when there are less clouds and winds are light (Figure 2). Over exposed inland areas in the New Territories such as Ta Kwu Ling and Shek Kong, there are no tall buildings to trap the heat during the day and block the release of heat at night. They are also far away from the coast so that temperature will not be moderated by the air over warmer sea. Consequently, radiation cooling is more prominent and temperatures are lower over inland areas, when compared with coastal areas. For the same reason, air temperatures over exposed inland areas rise more quickly during daytime, leading to a large temperature difference between day and night.
Figure 2 At clear night, temperature drops as the ground releases heat to the sky in the form of infrared radiation.
However, if there are more clouds at night, the temperature drop will not be that significant even over exposed rural area. This is because cloud layer, acting like a blanket, blocks and absorbs part of the heat transferred from the ground to the sky, and then re-emits it to the ground (Figure 3). Similarly, moist air near the ground can also slow down the cooling of the ground. Furthermore, strong winds facilitate the mixing of warmer air aloft and cooler air near the ground, slowing down the cooling process as well.
Figure 3 Cloud layer blocks and absorbs part of the heat transferred from the ground to the sky, and then re-emits it to the ground.
Besides radiation cooling, the complex landscape in Hong Kong also contributes to regional temperature difference. Let us take Pak Tam Chung weather station, which is situated in a valley, as an example. Cooler air over the hillsides at night sinks along the slope due to its higher density and accumulates in the valley. As a result, temperature recorded at Pak Tam Chung could be a few degrees lower than that recorded at neighbouring weather station such as Sai Kung. We can see from this example that air temperature depends on a number of meteorological and local geographical factors, and may differ considerably between places just several kilometres apart.
Now being in December, weather has already become cold. If you live in exposed area in the New Territories and there is a fine and dry day with light winds, remember to keep yourself warm at night so as not to get a cold. Furthermore, as temperatures can vary significantly across different parts of Hong Kong, the Observatory will soon launch an automatic regional weather forecast to complement the territory-wide weather forecast. Stay tuned for this new service and obtain the weather information that best suits your needs.